linux-kernel - Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

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Message-ID: <20210330171041.70f2d7d0@jacob-builder>
Date:   Tue, 30 Mar 2021 17:10:41 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>,
        iommu@...ts.linux-foundation.org, cgroups@...r.kernel.org,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
        Wu Hao <hao.wu@...el.com>, Dave Jiang <dave.jiang@...el.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and
 allocation APIs

Hi Jason,

On Tue, 30 Mar 2021 10:43:13 -0300, Jason Gunthorpe <jgg@...dia.com> wrote:

> > If two mdevs from the same PF dev are assigned to two VMs, the PASID
> > table will be shared. IOASID set ensures one VM cannot program another
> > VM's PASIDs. I assume 'secure context' is per VM when it comes to host
> > PASID.  
> 
> No, the mdev device driver must enforce this directly. It is the one
> that programms the physical shared HW, it is the one that needs a list
> of PASID's it is allowed to program *for each mdev*
> 
This requires the mdev driver to obtain a list of allowed PASIDs(possibly
during PASID bind time) prior to do enforcement. IMHO, the PASID enforcement
points are:
1. During WQ configuration (e.g.program MSI)
2. During work submission

For VT-d shared workqueue, there is no way to enforce #2 in mdev driver in
that the PASID is obtained from PASID MSR from the CPU and submitted w/o
driver involvement. The enforcement for #2 is in the KVM PASID translation
table, which is per VM.

For our current VFIO mdev model, bind guest page table does not involve
mdev driver. So this is a gap we must fill, i.e. include a callback from
mdev driver?

> ioasid_set doesn't seem to help at all, certainly not as a concept
> tied to /dev/ioasid.
> 
Yes, we can take the security role off ioasid_set once we have per mdev
list. However, ioasid_set being a per VM/mm entity also bridge
communications among kernel subsystems that don't have direct call path.
e.g. KVM, VDCM and IOMMU.

> > No. the mdev driver consults with IOASID core When the guest programs a
> > guest PASID on to he mdev. VDCM driver does a lookup:
> > host_pasid = ioasid_find_by_spid(ioasid_set, guest_pasid);  
> 
> This is the wrong layering. Tell the mdev device directly what it is
> allowed to do. Do not pollute the ioasid core with security stuff.
> 
> > > I'd say you shoul have a single /dev/ioasid per VM and KVM should
> > > attach to that - it should get all the global events/etc that are not
> > > device specific.
> > >   
> > You mean a single /dev/ioasid FD per VM and KVM? I think that is what we
> > are doing in this set. A VM process can only open /dev/ioasid once, then
> > use the FD for allocation and pass the PASID for bind page table etc.  
> 
> Yes, I think that is reasonable.
> 
> Tag all the IOCTL's with the IOASID number.
>  
> > > Not sure what guest-host PASID means, these have to be 1:1 for device
> > > assignment to work - why would use something else for mdev?
> > >   
> > We have G-H PASID translation. They don't have to be 1:1.
> > IOASID Set Private ID (SPID) is intended as a generic solution for
> > guest PASID. Could you review the secion Section: IOASID Set Private ID
> > (SPID) in the doc patch?  
> 
> Again this only works for MDEV? How would you do translation for a
> real PF/VF?
> 
Right, we will need some mediation for PF/VF.

> So when you 'allow' a mdev to access a PASID you want to say:
>  Allow Guest PASID A, map it to host PASID B on this /dev/ioasid FD
> 
> ?
> 
Host and guest PASID value, as well as device info are available through
iommu_uapi_sva_bind_gpasid(), we just need to feed that info to mdev driver.

> That seems like a good helper library to provide for drivers to use,
> but it should be a construct entirely contained in the driver.
why? would it be cleaner if it is in the common code?

Thanks,

Jacob